High power radio frequency (RF) transmission signals can have sufficient energy to make it desirable to provide safety equipment to reduce personnel exposure risk to the RF signals. Safety equipment to protect against excessive exposure to RF signals exists in many forms, including the use of switches and mechanical locks to keep transmitting equipment deenergized until the technician who applied the locks removes them, as well as many types of guards and other shields to protect a technician who may be near a radiation-capable mode when RF energy is inadvertently present.
High power RF signals are normally carried from a transmitter to a radiating antenna using either a relatively large-diameter—e.g., 1 inch to 1 foot—rigid coaxial line for frequencies below ultra-high frequency (UHF), or a waveguide for UHF and above. Waveguides, commonly rectangular or circular in cross section, are critically dimensioned with respect to frequency; specifically, a waveguide must be of at least a certain minimum size to carry RF signals of a certain frequency. The lower the frequency, the larger the waveguide must be; below the UHF television transmission band, waveguides can become too large (e.g., heavy, creating excessive wind drag in a feedline that extends up a transmission tower, and disproportionately expensive to manufacture) to be practical for many purposes. In the UHF band, however, waveguides are quite practical, even for signals of more than a megawatt.
Waveguides in general can provide distinct advantages when compared to a coaxial line. A coaxial line is limited in the power it can carry, because current flows in the conductors making up the line. Heating associated with that current flow becomes unacceptably large as power increases, requiring a larger coaxial line. Moreover, as frequency increases, depth of penetration of the current in the conductors decreases due to skin effect, so the increasing current flows in a decreasing volume of conductor, again dictating an increase in size. In addition to weight and wind load issues, very large coaxial lines also permit waveguide propagation modes; with different propagation rates, these cause severe distortion as well as damaging reflections.
Waveguides, as the name implies, provide primarily an environment in which the RF signal propagates. While there are losses (that is, conversion of RF signals to heat) associated with use of waveguides, the limit to the power level that can be carried is defined in terms of voltage peaks that cause arcing across the narrow dimension of the waveguide. For cases where equivalent weights and wind loadings exist, waveguides can carry significantly greater power than comparable coaxial lines. Moreover, as frequency increases, the measured loss in a given size of waveguide decreases. Thus, while the power level determines the size of a coaxial line the frequency of the signal determines the size of a waveguide.
Rectangular waveguides are normally operated in the fundamental mode for RF signal stability. This means that no signal with a frequency below cutoff, and thus a wavelength longer than the broad dimension of the waveguide, can propagate in a waveguide of a particular size. The Electronics Industry Association (EIA) specifies that for normal use, a waveguide should be half the size on the narrow axis that it is on the broad axis, which reasonably guarantees that the waveguide cannot spontaneously switch to the orthogonal propagation mode. Circular waveguides, which can more readily switch modes, require more care in application, but are still usable with switches and safety devices using the inventive apparatus.
Radiated UHF signals tend to be available for reception over a shorter distance from a transmitting antenna than signals at lower frequencies, such as very high frequency (VHF), which, in the U.S., includes the low-numbered television channels, 2 through 13, and the FM radio broadcast band. This is because UHF energy is attenuated more readily in the atmosphere and propagates strictly by line of sight. As a consequence, UHF broadcasters seeking to provide comparable reception quality over a comparable area are obliged to use higher power levels than VHF broadcasters, which increases the energy level in the system while it is energized. Representative power levels are 30 kilowatts (KW) for VHF and 200 KW for UHF. Such high energy levels represent intrinsic hazards for which good safety equipment is desirable.
Accordingly, there is a need in the art for a mechanical switch that can cause impinging RF signals in a waveguide to be at least substantially blocked from emission. It would also be desirable if such a switch could be further applied to provide a low-loss switch for directing RF signal flow in a high-power waveguide environment.